1. Field of the Invention
The present invention relates to a viscoelastic substance and an objective lens driving apparatus used in an optical disk apparatus and provided with the substance.
2. Description of the Related Art
Optical disk apparatuses, for use as optical information processing apparatuses, are characterized by their information storage capacities much greater than those of conventional magnetic recording apparatuses, and are presently used in various fields of application. Principally, these optical disk apparatuses have modern technical problems that they should be able to access desired information quickly and accurately, and to offer the information to users without delay. Many technical improvements have already been made on electromagnetic drive elements (e.g., voice-coil motor formed of a coil and a magnetic circuit) for driving the objective lens, so that the lens can enjoy a sufficient driving force.
On the other hand, the objective lens, which can be moved by means of such a sufficient driving force, must be positioned (or stopped) quickly and accurately at a predetermined position. This is a second requirement for the quick and accurate access to the information on the optical disk. More specifically, fine residual vibrations produced in the objective lens must be removed for the quick and accurate positioning of the lens. As regards this requirement, however, there still is much room for improvement, and various investigations are presently being made on the removal of fine vibrations.
According to one of conventional methods for removing fine vibrations of the objective lens, a magnetic fluid is interposed between a coil on the movable side and a magnetic circuit on the fixed side. In this arrangement, the viscosity resistance of the magnetic fluid accelerates a damping effect between movable and fixed parts, so that fine vibrations of the movable part can be absorbed at once. Due to the influence of a magnetic field generated by the magnetic circuit, moreover, the magnetic fluid can stay in magnetic gaps, so that the damping effect can be maintained for a long period of time.
Using the magnetic fluid, however, a conventional objective lens driving apparatus has the following problems. Since the magnetic fluid has a very great surface tension, it will gradually escape from the magnetic gaps if the gaps are relatively long. Moreover, the fluid may possibly evaporate or scatter during the operation of the optical disk apparatus. During prolonged use, therefore, the magnetic fluid is also likely to escape gradually from the magnetic gaps. Such an escape of the magnetic fluid is also attributable to a relatively small magnetic force of the magnetic circuit which is designed for the drive of the objective lens.
Thus, with use of the magnetic fluid, a prolonged, steady vibration damping effect cannot be expected. Since the viscosity of the magnetic fluid greatly changes depending on a temperature change, furthermore, the vibration damping effect also changes. It is therefore difficult to use the magnetic fluid within a practical range of working temperature (-15 to +60.degree. C.). Thus, the objective lens driving apparatus cannot easily effect control for the quick and accurate access to the target information.